Stabilisation after Fukushima cooling change

Cooling of Fukushima Daiichi unit 2 has been upset by a change in injection rates, leading to a rise in temperature that Tokyo Electric Power Company (Tepco) is working to gradually correct.

Tepco has been injecting water into the reactor since 14 March 2011, first using seawater and changing to fresh water 12 days later. Over time, the accumulated decay heat produced by the reactor core was gradually removed. By September 2011, Tepco had improved the water injection method to include core spray systems as well as feedwater injection, and temperatures fell below the landmark of 100ºC - and eventually to about 50ºC.

This stability of unit 2 was disturbed for a few days, however, when Tepco tried to improve cooling further by tuning the rates of water injection. On 2 February, feedwater injection was reduced by two cubic metres per hour and the core spray was stepped up by the same amount.

Water injection from the dark blue feedwater line was reduced, while the light blue core spray was increased by the same amount

Readings from the three sensors in the reactor vessel bottom head of Fukushima Daiichi 2 (Images: Tepco)

After making this change, Tepco noted a tendency for increasing temperature at the bottom of the reactor vessel. Within a matter of hours the company decided to reverse the change and restore the previous injection rates, but the temperature continued to slowly rise.

Two of the three temperature sensors at the bottom of the reactor vessel edged up by about 2ºC. The third, however, rose by around 20ºC to hit 72.2 degrees at 5.00am today. Tepco acted to stem this increase by injecting an extra cubic metre of water per hour through the feedwater line, and this stabilised the sensor at about 70ºC. It has since decreased to 68.5ºC, while the other two sensors were at a new low of around 41ºC.

Tepco was able to discount recriticality as a potential cause of the temperature rise after conducting an analysis of charcoal filters in the containment gas control system. These showed very low traces of fission products that were below the threshold that would indicate criticality. Nevertheless Tepco this morning injected boric acid into the reactor vessel as a precaution and increased the core spray injection rate by three cubic metres per hour.

The majorty of unit 2's core is thought to have melted and slumped to the bottom of the reactor vessel, but its configuration there is unknown. What Tepco's experience indicates is that one portion of the deformed core relies on a certain flow from the feedwater pipe for cooling. While the core spray complements feedwater input and was significant in the push to bring the temperature below 100ºC, the two are apparently not interchangeable for one specific area of unit 2's core. Tepco has modified injection rates at all three of the melted Fukushima Daiichi reactors several times in the past without experiencing warming effects such as this.

The current injection rates are 6.8 cubic metres per hour through the feedwater system, and 6.7 cubic metres per hour through the core spray. Tepco continues to report the status of the Fukushima Daiichi reactors every few hours, as they have done since the natural disasters of 11 March last year.